In the past, a wave field synthesis technique has been known in which a sound is picked up on a wave surface of the sound in a sound field using a plurality of microphones to reproduce the sound field on the basis of a sound pickup signal that has been obtained.
For example, in a case where a sound field within a closed space is required to be accurately reproduced, it is possible to reproduce the sound field according to Kirchhoff-Helmholtz theory in which sound pressure at a boundary surface of the closed space and sound pressure gradients at all coordinates within the closed space are recorded and then, sounds are played back at corresponding coordinates using a sounding body having a dipole property and a sounding body having a monopole property.
In a real environment, a microphone and a speaker are used to record and play back the sound field. Typically, a simple pair of a microphone for sound pressure and a monopole speaker is used due to physical restriction. In this case, a difference is generated between a played-back sound field and an actual sound field because of a lack of sound pressure gradients.
As a representative example where such a difference is generated, a case is given in which a signal arriving from a sound source at the outside of the closed space and a signal arriving from the inside of the closed space due to another sound source at the outside of the closed space by passing through the interior of the closed space are mixed when recorded. As a result, in this example, the two sound sources are heard from unexpected positions during playback. In other words, positions of the sound sources perceived by a user hearing the sound field are set at positions different from original positions at which the sound sources should be located.
This phenomenon is caused by a signal that has been originally canceled out in a physical manner in a listening area corresponding to the closed space is maintained due to a lack of acquiring the sound pressure gradients.
Therefore, for example, a technique has been proposed in which a microphone is arranged at a surface of a rigid body to make the sound pressure gradient zero, thereby solving the occurrence of the aforementioned phenomenon (for example, refer to Non-patent Document 1).
In addition, another technique has been also proposed in which the boundary surface of the closed space is limited to a flat surface or a straight line to exclude the influence of the signal arriving from the inside of the boundary surface, thereby preventing the aforementioned phenomenon from occurring (for example, refer to Non-patent Document 2).